1. Field of the Invention
The present invention relates to a digital car telephone and cellular phone system (cellular system) using a CDMA (Code Division Multiple Access) communication scheme, and more particularly to a CDMA communication device and a transmission power control method.
2. Description of the Related Art
In recent years, the CDMA communication scheme attracts attention as a communication method for use in mobile communication systems due to its resistance to interference and disturbance. The CDMA communication scheme refers to a communication scheme in which a transmitting side spreads a user signal to be transmitted with a spreading code and then transmits the signal, while a receiving side performs despread with the same code as that used by the transmitting side to obtain the original user signal.
In the CDMA communication scheme, a plurality of transmitting sides perform spread with different spreading codes each having orthogonality, and a receiving side selects a spreading code for use in despread to allow each communication to be specified, thereby making it possible for a plurality of communications to use the same frequency band.
However, since it is difficult to completely maintain the orthogonality among all the spreading codes being used, exact orthogonality is not obtained actually in the respective spreading codes, resulting in a correlational component between one code and another code. The correlational component serves as an interference component for the communication associated with the one code and contributes to deterioration of communication quality. Since the interference component is produced from such a factor, the interference component is increased as the number of the communications is increased.
For this reason, there is a critical risk in that, when signal power from one mobile station is significantly higher than signal power from another mobile station at an antenna of a base station, the base station can not demodulate the signals other than the signal from the mobile station which produces the high received power, of a plurality of signals received at its antenna. The problem is widely known as a near-far problem in the CDMA communication scheme. In the CDMA cellular system, to solve the near-far problem, a TPC (Transmission Power Control) technique is essential in which the signal power from all transmitters at an input of a receiver is controlled to have the same magnitude.
In the transmission power control, a receiver has a preset Eb/N0 (desired wave power density to noise power density ratio) serving as a reference and the receiver controls transmission power from each transmitter such that an Eb/N0 obtained from a communication of each transmitter is equal to the Eb/N0 reference value. More specifically, a downward channel for transmission from a base station to a mobile station includes a transmission power control signal for directing an increase or decrease in transmission power of the mobile station, which data is used by the base station to direct an increase or decrease in current transmission power to each mobile station. On the other hand, an upward channel for transmission from the mobile station to the base station includes a transmission power control signal for directing an increase or decrease in transmission power to the base station, which data is used by the mobile station to direct an increase or decrease in current transmission power to the base station.
In the CDMA cellular system, a rake/diversity reception technique is also essential to high-quality reception under a multipath fading environment.
FIG. 1 is a block diagram showing a configuration of a mobile station in such a cellular system.
The mobile station comprises n antennas 1011 to 101n, transmit/receive amplifier (AMP) 102, radio unit (TRX) 103, baseband signal processor (BB) 104, control unit (MS-CONT) 105, and terminal interface unit (TERM-INT) 106.
Antennas 1011 to 101n transmit an upward RF signal amplified at a transmission amplifier in transmit/receive amplifier 102, and receive a downward RF signal from a base station and output the signal to transmit/receiver amplifier 102. Diversity reception is achieved by using n antennas 1011 to 101n. Transmit/receive amplifier 102 is provided with a transmission amplifier for amplifying a transmission RF signal and a low noise amplifier for amplifying a reception RF signal, and demultiplexes the RF transmission signal and the RF reception signal for connection to antennas 1011 to 101n. Radio unit 103 converts a transmission signal which has been subjected to baseband spread from digital form to analog form and performs quadrature modulation for conversion to an RF signal, and performs quasi-synchronous detection and converts a signals received from transmit/receive amplifier 102 from analog form to digital form for transmission to baseband signal processing unit 104.
Baseband signal processing unit 104 performs baseband signal processing such as error correction coding for transmission data, framing, data modulation, spread modulation, despread of a received signal, chip synchronization, error correction decoding, data demultiplexing, diversity handover combination function, reception level measuring function, and the like.
Control unit 105 performs control of the entire mobile station. Terminal interface unit 106 has a function as an adapter for voice and various types of data, as well as a function as an interface to a handset and image/data terminals.
Next, FIG. 2 shows a configuration of a conventional CDMA communication device provided in baseband signal processing unit 104.
The conventional CDMA communication device comprises n rake receivers 2021 to 202n, mixer 204, determinator 205, n Eb/N0 measuring units 3061 to 306n, transmission power control information generator 307, rake receiver control unit 208, and multipath searcher 209.
As shown in FIG. 3, rake receiver 2021 includes m finger receivers 501 to 50m and mixer 70. Since the configurations of rake receivers 2022 to 202n are similar to that of rake receiver 2021, the description thereof is omitted.
Multipath searcher 209 acquires, from received signals 101 to 10n, reception delay information which is information on delay times among respective paths contained in received signals 101 to 10n. Rake receiver control unit 208 sets delay times for finger receivers 501 to 50m in each of rake receivers 2021 to 202n based on the reception delay information acquired at multipath searcher 209.
Rake receivers 2021 to 202n detects received signals 1011 to 101n, and output demodulated data which is the result of the detection. Specifically, finger receivers 501 to 50m generate the demodulated data by individually demodulating received signal 101 for respective paths, delay the demodulated data using the delay time set by rake receiver control unit 208, and then output the data to mixer 70. Mixer 70 combines the demodulated data from finger receivers 501 to 50m and outputs the combined data.
Mixer 204 combines the demodulated data from rake receivers 2021 to 202n into one signal. Determinator 205 performs decoding with a determination of the signal combined by mixer 204 for output as decoded signal 20.
Eb/N0 measuring units 3061 to 306n measure the Eb-N0s of the signals from rake receivers 2021 to 202n. Transmission power control information generator 307 sums the Eb/N0 measured at Eb/N0 measuring units 3061 to 306n to obtain the measurement result of the Eb/N0s, and generates transmission power control information 30 based on the measuring result of the Eb/N0s. Transmission power control information 30 is transmitted to the base station which is a transmitter through an upward channel, thereby performing transmission power control.
Next, the operation of the conventional CDMA communication device is described. In the conventional CDMA communication device, received signals 101 to 10n input thereto are sent to multipath searcher 209 which acquires reception delay information which is information on delay times among respective paths contained in received signals 101 to 10n. Rake receiver control unit 208 sets delay times for finger receivers 501 to 50m in each of rake receivers 2021 to 202n based on the reception delay information acquired at multipath searcher 209. Received signals 101 to 10n demodulated and delayed by finger receivers 501 to 50m are combined by mixer 70 as demodulated data after detection.
Mixer 204 combines the demodulated data obtained at rake receivers 2021 to 202n into one signal. The demodulated data combined at mixer 204 is subjected to a determination at determinator 205, resulting in decoded signal 20.
The demodulated data obtained at rake receivers 2021 to 202n are input to Eb/N0 measuring units 3061 to 306n for measuring the Eb/N0s of the respective demodulated data. Transmission power control information generator 307 derives the measuring result of the Eb/N0s by summing up the respective measured Eb/N0s, and generates transmission power control information 30 based on the obtained measuring result of the Eb/N0s.
In the mobile station in the conventional CDMA cellular system, the measuring result of the Eb/N0s is obtained by measuring the Eb/N0s for respective rake receivers 2021 to 202n and summing up the measured Eb/N0s. However, under an environment with fading, interference of another signal, reception of highly correlated signals associated with a reception delay, or the like, the accuracy of the Eb/N0 measurement is deteriorated to cause the inability to perform stable transmission power control.
Additionally, in the conventional CDMA cellular system, the n demodulated data obtained from rake receivers 2021 to 202n are combined as they are. For this reason, in some reception states, the Eb/N0 of the demodulated data after the combination obtained by mixer 204 may not be higher than the Eb/N0 of the demodulated data before the combination.
It is an object of the present invention to provide a CDMA communication device and a transmission power control method thereof capable of performing stable transmission power control even in a deteriorated reception state.
According to a first aspect of the present invention, the CDMA communication device comprises means for weighting outputs from a plurality of rake receivers, respectively, and means for measuring the Eb/N0 after weighted signals are combined.
In the present invention, since demodulated data from the respective rake receivers are combined after the weighting, the Eb/N0 of the demodulated data after combination is not deteriorated as compared with the Eb/N0 of the demodulated data before the combination. In addition, since the Eb/N0 is measured using the demodulated data after the combination, the Eb/N0 can be measured with higher accuracy even under a deteriorated reception environment.
In an embodiment of the present invention, the CDMA communication device further comprises means for weighting based on RSSI of an output from each rake receiver.
In another embodiment of the present invention, the CDMA communication device further comprises means for controlling transmission power based on the measured Eb/N0.
According to a second aspect of the present invention, the CDMA communication device comprises means for weighting outputs from a plurality of rake receivers, and combines the weighted signals and controls transmission power based on the combined signal.
Since the present invention performs transmission power control with the Eb/N0 derived using the demodulated data after the combination, more accurate transmission power control is possible to allow stable communication.
According to a third aspect of the present invention, the CDMA communication device comprises a plurality of rake receivers, a plurality of first weighting units, a first combining unit, and an Eb/N0 measuring unit.
Each of the plurality of rake receivers demodulates a received signal for each path and combines the respective demodulated data after the respective data are delayed based on a delay time. The plurality of first weighting units weight the demodulated data after the combination output from the respective rake receivers. The first combining unit combines the signals weighted by the first weighting units. The Eb/N0 measuring unit measures an Eb/N0 of the demodulated data combined by the first combining unit.
According to a fourth aspect of the present invention, the CDMA communication device for decoding a plurality of received signals received at different antennas comprises a plurality of rake receivers, a multipath searcher, a rake receiver control unit, a plurality of first weighting units, a first combining unit, and an Eb/N0 measuring unit.
Each of the receivers detects each of the received signals for each path, delays demodulated data which are the detection result based on a set delay time, and then combines and outputs the demodulated data. The multipath searcher acquires reception delay information which is information on delay times among respective paths contained in the received signals. The rake receiver control unit sets the delay time for the respective rake receivers based on the reception delay information acquired by the multipath searcher. The plurality of first weighting units weight the demodulated data output from the respective rake receivers. The first combining unit combines the signals weighted by the first weighting units into one signal. The Eb/N0 measuring unit measures an Eb/N0 of the demodulated data combined by the first combining unit.
Since the present invention performs the Eb/N0 measurement with the demodulated data after the combination by the first combining unit, more accurate Eb/N0 measurement is possible even under a deteriorated reception environment.
In addition, the present invention utilizes the first weighting units to weight the demodulated data from the respective rake receivers before the combination by the first combining unit. Thus, the Eb/N0 of the demodulated data after the combination is not deteriorated as compared with the Eb/N0 of the demodulated data before the combination.
Furthermore, in the present invention, the Eb/N0 measurement with the demodulated data combined by the first combining unit requires only one Eb/N0 measuring unit, thereby allowing a simplified configuration of the CDMA communication device.
According to an embodiment of the present invention, each of the receivers includes a plurality of finger receivers, a plurality of second weighting units, and a second combining unit.
The plurality of finger receivers generate demodulated data by individually demodulating a received signal for each path and then output the demodulated data after the demodulated data are delayed with delay time information contained in the received signal. The plurality of second weighting units weight the demodulated data output from the finger receivers, respectively. The second combining unit combines the demodulated data from the second weighting units and outputs the combined data.
According to the present invention, the second weighting units are provided between the respective finger receivers and the second combining unit, and weighting can also be performed individually for the demodulated data output from the respective finger receivers, thereby enabling estimation for each transmission path.
According to another embodiment of the present invention, the CDMA communication device further comprises means for controlling transmission power based on the Eb/N0 measured by the Eb/N0 measuring unit.
Since the present invention controls transmission power with the Eb/N0 derived with the demodulated data after the combination, more accurate transmission power control is possible to allow stable communication.
According to a fifth aspect of the present invention, the CDMA communication device comprises a transmission power control information generator for generating transmission power control information which is information for controlling transmission power based on the Eb/N0 measured by the Eb/N0 measuring unit.
The present invention generates the transmission power control information with the Eb/N0 derived using the demodulated data after the combination, thereby making it possible to perform more accurate transmission power control and stable communication.